Why does my boolean keep flickering?
Flickering of surfaces within the 3DView is caused by overlapping faces. With a boolean modifier you get the new generated faces in the same location as the mesh used to perform the cut, leading to flickering.

Having a mirror modifier on a mesh that has faces on both sides of the mirrored centre line can also lead to flickering.

If you want to keep the boolean modifier and not apply it, you can place the cutting object onto another layer (M and select a layer) that is not active or you can hide the object from view (H)



You can also use the outliner to hide/unhide objects.



The eye for each item specifies whether it is visible in the 3DView, the cursor allows you to select the item in the 3DView and the camera is whether it shows up in the final render. Click any of these icons to turn them on/off.

If you have overlapping faces from a mirror modifier then you need to remove the duplicate faces. This may involve deleting everything on one side of the mesh or just the few faces that have overlapped. The mirror modifier has a clipping option that can help line up vertices on the centre line of your mesh.